Microwave heating is a well-developed technology, and microwave oven is a kind of popular heating appliance. Microwave oven heats food by irradiating in the cooking chamber of the oven, an electromagnetic radiation in the microwave spectrum and causes polar molecules (e.g. water) in the food to rotate and build up thermal energy in a process known as the dielectric heating. A traditional microwave oven uses 2.45 GHz as a fixed working frequency. Also known microwave ovens using multiple fixed frequencies via the use of high-power traveling wave tubes, the multiple frequencies are determined manually by user.
However, it appears that the fixed heating frequency used in known microwave ovens is not an optimal choice in terms of heating.
US2013142923A1 discloses a method of processing objects by radio frequency (RF) energy. The method comprises heating the object by applying radio frequency energy, monitoring a value related to a rate of absorption of RF energy by the object during the heating, and adjusting the RF energy in accordance with changes in a time derivative of the monitored value. However, this known method also does not always achieve an optimal heating of the food ingredients.
EP2434837A1 discloses a microwave heating device that is configured to prevent a microwave generating part from being destroyed by reflected power. The microwave heating device comprises a control part that executes a frequency sweep operation of a prescribed frequency band to set an oscillation frequency at which the minimum reflected power becomes minimum and to control the oscillation frequency of an oscillator part and the output of a power amplifier part.
US2009057302A1 discloses heating a load such as food in a resonant cavity by RF radiation. The RF feed sweeps over a plurality of frequencies, determining at each frequency the absorption of the radiation energy. The food is then radiated at frequencies that optimize heating goals, for example, corresponding to the most efficient absorption frequencies to achieve faster heating.